Ion source having an expansion cup for effecting beam divergence

Claim 1

AN ION SOURCE COMPRISING AN IONIZATION CHAMBER, MEANS FOR SUPPLYING THERETO A QUANTITY OF GAS TO BE IONIZED, MEANS FOR IONIZING SAID GAS, AN EVACUATED PLASMA FORMING CHAMBER, AN APERTURED ANODE MEMBER, AN EXTRACTOR ELECTRODE AT A HIGH POTENTIAL WITH RESPECT TO THAT OF SAID ANODE MEMBER, SAID ANODE MEMBER COMPRISING A COMMON PARTITION BETWEEN SAID IONIZATION CHAMBER AND SAID EVACUATED PLASMA FORMING CHAMBER, SAID ANODE MEMBER HAVING AN APERTURE THEREIN OF A SIZE COMMENSURATE TO THE ION BEAM CURRENT REQUIREMENTS OF SAID ION SOURCE, SAID ION SOURCE PRODUCING A CONCENTRATION OF IONS AT SAID APERTURE SUFFICIENTLY GREAT SO THAT THERE WOULD BE A TENDENCY FOR SPACE CHARGE REPULSION TO PRODUCE A HIGHLY DIVERGENT BEAM UPON EXTRACTION OF POSITIVE IONS AT THAT POINT, A CYLINDRICAL MEMBER CONTIGUOUS TO AND COAXIALLY ALIGNED WITH THE APERTURE IN SAID ANODE MEMBER AND PROTRUDING INTO SAID ION BEAM FORMING CHAMBER, SAID CYLINDRICAL MEMBER BEING AT THE SAME ELECTRICAL POTENTIAL AS THAT OF SAID ANODE MEMBER AND BEING OF SUFFICIENT LENGTH OF FORM A REGION FREE OF INFLUENCE OF THE FIELD EFFECT OF THE EXTRACTOR ELECTRODE UNTIL SUCH TIME AS SAID PLASMA HAS DIFFUSED TO A SUFFICIENT EXTENT TO REDUCE SPACE CHARGE EFFECTS IN THE ION BEAM TO BE EXTRACTED THEREFROM, SAID CYLINDRICAL MEMBER THUS PROVIDING A DRIFT SPACE ADAPTED TO ALLOW EXPANSION OF PLASMA PASSING THERETHROUGH AND HAVING THE SHAPE OF A FRUSTUM WHOSE DIAMETER INCREASES IN THE DIRECTION AWAY FROM SAID PLASMA FORMING CHAMBER, SO AS TO ESTABLISH A UNIFORM CONCAVE PLASMA BOUNDARY THEREIN, AND A FOCUSING ELECTRODE.